SI540 Understanding Networked Computing

Internet Protocols

Midterm is two weeks away Break is Saturday

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Review

• Given a video file with a message latency of 2 minutes, how much faster will a streamed version of the video start to play if each video segment has a message latency of 4 seconds?

• Suppose that a video is divided into 2-second segments for a streaming broadcast› Does streaming work if each segment has a 1 second

transmission time and a 3 second delay?› Does streaming work if each segment has a 3 second

transmission time and a 1 second delay?

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Fetching a Web Page

www.si.umich.edu

Home computer

The Internet

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Fetching a Web Page

www.si.umich.edu

Home computer

The Internet

MichNet Modem

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Overview Of Internet Topics

• Understand and explain general principles…› Layered protocols› Indirection in naming› Packet routing› Local and hierarchical namespaces› End-to-end principle

• …As applied to Internet› IP/TCP/HTTP› DNS and IP addresses› Pressures for Internet evolution

• Understand structure of ISP industry• Diagnose connection troubles

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Learning Objectives

• Understand the Internet protocol stack• Understand what IP provides and how it provides it

› Understand IP addresses & packet routing

• TCP• More advanced topics

› The end-to-end principle› Quality of service› Pricing

• HTTP• Web server administration

Future sessions

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Simplified Internet Topology

Hosts

Routers/Switches

Access links

Backbonelinks

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Terminology

• A network is the collection of routers, communication lines and hosts controlled by a network operator (or an organization or a homeowner)› In the diagram, these are the matching yellow

ovals and white lines that connect them

• An internet is a network of networks• The Internet is the public network of

networks we use all the time

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Internet Protocol (IP)

Subnets

User Datagram Protocol (UDP)

Transmission Control Protocol (TCP)

HyperText Transport Protocol (HTTP)

Application

Real-time Transport Protocol (RTP)

Conceptual Layering of Internet Protocols/Services

Application Layer

Transport Layer

Network Layer

Data Link and Physical Layer

Internet Protocol (IP)

Last week’s topic

Today’s topic

Each layer provides services to the layer above, and utilizes

service provided by the layer below

Each layer provides services to the layer above, and utilizes

service provided by the layer below

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Comparing the OSI and TCP/IP Reference Models

Data Link

Physical

Network

Transport

Presentation

Session

Application

Host-to-Network

Transport

Application

Internet

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Questions

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Network/Internet Layer

• Responsible for routing packets from source to destination› Not responsible for the packet’s payload

• IP is common› IPX (Novell Netware) is another

• Next we’ll discuss IP in terms of its › Service › Protocol

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IP Service

• Provides best effort packet delivery› Between two hosts › Not necessarily sharing a common LAN or

subnet

• How› Global addressing› Packet forwarding

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IP Protocol

• The IP protocol specifies three things› IP packet format› IP addresses› IP packet routing

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IP Protocol: Packet Format

• Header› Source IP address› Destination IP address› HopLimit› Payload length

• Payload› Actual data Header Payload

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IP Protocol: Addresses

• Every host gets a distinct address› Can be dynamically assigned

• IPv4 (currently ubiquitous)› Each address 32 bits› Divide into 8 bit segments› Example: 141.211.203.32› 4 billion addresses

• IPv6 (future widespread adoption?)› Each address 128 bits› “1500 addresses per square foot”

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IP Address Assignment

• Fixed (static) IP address› Computer always has same IP address

• Dynamic IP address› Address changes each time computer connects to

network› Internet Access Provider (U-M, AOL, MSN, etc.)

assigns an address from its pool› Uses DHCP to allocate addresses› All addresses in that pool are routed to the provider› Provider forwards on to correct final destination

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Questions

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IP Protocol: Routing

• Final destination IP address written in packet› Not the full route

• Each router has connections to several hosts• Each router keeps a table that indicates

where to go based on final destination

• Reducing table size› use wildcards: 141.211.* next hop is X

Final Destination Next hop

141.211.203.032 X

207.075.186.001 Y

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IP Routing Demonstration #1

• Envelope is an IP packet› Inside is the payload (a bit string)› Outside specifies a destination in the format (subnet, ID)

• If you have two packets, drop the second one• Consult your routing table

› Routers with ID of 0 can pass packets to different subnets› All other routers must pass to router at ID 0 if packet needs

to reach another subnet› Routers send packets on destination subnet to correct ID› If destination subnet and ID match your address, open the

envelope

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Routing Changes

• Routers talk to each other (e.g., BGP protocol)› Advertise routes

› “I’m now accepting traffic for 141.211.*”

› Query for route availability› “Is anyone accepting traffic for 141.211.*?”

• Update entries in own routing table

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Routing Dangers

• Long routes• Circular routes

› Use “HopLimit” to limit damage› Decrement HopLimit at each router› Discard packet if HopLimit=0

• Hijacking routes› Advertise a route, but don’t deliver

• Route flapping› Frequent updates to routing table

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IP Routing Demonstration #2:HopLimit

• Revised routing rules› Decrement HopLimit (cross out and write next smaller

number) › If HopLimit=0 then drop packet

› If you have two packets, drop the second one› Consult your routing table

› Routers with ID of 0 can pass packets to different subnets› All other routers must pass to router at ID 0 if packet

needs to reach another subnet› Routers send packets on destination subnet to correct ID› If destination subnet and ID match your address, open

the envelop

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Summary: Why Delivery Not Guaranteed

• Intermediate host not responding› Temporary malfunction› Queues full (congestion)

• Bad routing› Reach hop limit because

route was too long or circular

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Sharing an IP Address?

• DSL provider gives me one IP address

• I have several devices on my home network?› How many do you have?

• How does it work?

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Fetching A Web Page

www.si.umich.edu

Home computer

MichNet Modem

207.75.186.1

198.108.3.5 141.211.0.9

141.211.203.32

c-ccb2 router c-ugli router

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TRACERT SI.UMICH.EDU

1 * Request timed out. 2 140 ms 207.75.186.1 3 140 ms f-umbin.c-ccb2.umnet.umich.edu [198.108.3.5] 4 240 ms f-backbone.c-ugli.umnet.umich.edu [141.211.0.9] 5 141 ms bart.si.umich.edu [141.211.203.32]

www.si.umich.edu

Home computer

MichNet Modem

207.75.186.1

198.108.3.5 141.211.0.9

141.211.203.32

c-ccb2 router c-ugli router

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IP IP IP

Network 1 Network 2

TCP or UDP TCP or UDP

Host A Host B

Application Application

Switch or Router

N 2N 1

IP As Spanning Layer

Diversity

• A spanning layer is a common protocol offering consistent services and interfaces to the layers above it that has been implemented on a wide range of underlying networking technologies, such as Ethernet and token ring

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What IP Doesn’t Do

• Guarantee speed of delivery

• Guarantee delivery

• Guarantee order of delivery

• Maintain conversational context (each packet is independent)

• Specify a process that should handle the packet at destination

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Gateway

Public telephone network

Internet

Computer w/modem

IP over Voice

Computer w/modem

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Gateway

Public telephone network

Internet

IP telephone or computer running VoIP software

Plain oldtelephone

Voice over IP

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Summary

• The Internet protocol stack defines several protocol layers that work together to deliver Internet traffic

• IP provides best effort packet delivery using› Global addresses› Store-and-forward routers

• IP is a spanning layer› Available on a wide range of network architectures, with a

variety of applications built on top of it

• But there are lots of things that IP doesn’t do• Next time, TCP…

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Gateway

Public telephone network

Internet

Computer w/modem

IP over Voice

Computer w/modem

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Gateway

Public telephone network

Internet

IP telephone or computer running VoIP software

Plain oldtelephone

Voice over IP